Bowed rotor start using direct temperature measurement

ABSTRACT

A bowed rotor start mitigation system for a gas turbine engine is provided. The bow rotor start mitigation system includes a controller operable to receive a speed input indicative of a rotor speed of the gas turbine engine and a measured temperature of the gas turbine engine. The controller is further operable to drive motoring of the gas turbine engine by oscillating the rotor speed within a motoring band for a motoring time based on the measured temperature when a start sequence of the gas turbine engine is initiated.

BACKGROUND

This disclosure relates to gas turbine engines, and more particularly to an apparatus, system and method for mitigating a bowed rotor start condition using direct temperature measurement in a gas turbine engine.

Gas turbine engines are used in numerous applications, one of which is for providing thrust to an airplane. When the gas turbine engine of an airplane has been shut off for example, after an airplane has landed at an airport, the engine is hot and due to heat rise, the upper portions of the engine will be hotter than lower portions of the engine. When this occurs thermal expansion may cause deflection of components of the engine which may result in a “bowed rotor” condition. If a gas turbine engine is in such a “bowed rotor” condition it is undesirable to restart or start the engine.

Accordingly, it is desirable to provide a method and/or apparatus for detecting and preventing a “bowed rotor” condition.

BRIEF DESCRIPTION

In an embodiment, a bowed rotor start mitigation system for a gas turbine engine is provided. The bow rotor start mitigation system includes a controller operable to receive a speed input indicative of a rotor speed of the gas turbine engine and a measured temperature of the gas turbine engine. The controller is further operable to drive motoring of the gas turbine engine by oscillating the rotor speed within a motoring band for a motoring time based on the measured temperature when a start sequence of the gas turbine engine is initiated.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the motoring band includes a range of speeds below a resonance speed of the gas turbine engine.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the measured temperature is adjusted with respect to a measured ambient temperature of the gas turbine engine.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the motoring time is associated with a bowed rotor risk parameter that is determined based on the measured temperature.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the measured temperature is determined based on reading one or more temperature sensors of the gas turbine engine for a predetermined period of time when the start sequence of the gas turbine engine is initiated.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where based on determining that bowed rotor mitigation is complete, the controller is operable to monitor the vibration level while sweeping through a range of rotor speeds including a critical rotor speed and determine whether bowed rotor mitigation was successful prior to starting the gas turbine engine.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the measured temperature is determined based on reading data from one or more temperature sensors at station 3 of the gas turbine engine.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the measured temperature is determined based on reading data from one or more temperature sensors at station 4 of the gas turbine engine.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the measured temperature is determined based on reading data from one or more temperature sensors at station 4.5 of the gas turbine engine.

According to an embodiment, a gas turbine engine includes a motoring system operable to drive rotation of the gas turbine engine, a speed pickup, a temperature sensor, and an electronic engine control operable to receive a speed input from the speed pickup indicative of a rotor speed of the gas turbine engine and a measured temperature from the temperature sensor. The electronic engine control is further operable to drive motoring of the gas turbine engine by controlling the motoring system to oscillate the rotor speed within a motoring band for a motoring time based on the measured temperature when a start sequence of the gas turbine engine is initiated.

According to an embodiment, a method of bowed rotor start mitigation for a gas turbine engine includes receiving, by a controller, a speed input indicative of a rotor speed of the gas turbine engine. The controller also receives a measured temperature of the gas turbine engine. The controller drives motoring of the gas turbine engine by oscillating the rotor speed within a motoring band for a motoring time based on the measured temperature when a start sequence of the gas turbine engine is initiated.

A technical effect of the apparatus, systems and methods is achieved by using a start sequence for a gas turbine engine as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the present disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a gas turbine engine;

FIG. 2 is a schematic illustration of a starting system for a gas turbine engine in accordance with an embodiment of the disclosure;

FIG. 3 is a schematic illustration of a starting system for a gas turbine engine in accordance with another embodiment of the disclosure;

FIG. 4 is a block diagram of a system for bowed rotor start mitigation in accordance with an embodiment of the disclosure;

FIG. 5 is a flow chart illustrating a method of bowed rotor start mitigation using direct temperature measurement of a gas turbine engine in accordance with an embodiment of the disclosure;

FIG. 6 is a graph illustrating a bowed rotor risk score with respect to time in accordance with an embodiment of the disclosure;

FIG. 7 is a graph illustrating starting profiles of a normal start and a bowed rotor mitigation sequence start in accordance with an embodiment of the disclosure;

FIG. 8 is a graph illustrating examples of various vibration level profiles of an engine in accordance with an embodiment of the disclosure;

FIG. 9 is a schematic illustration of a high spool gas path with a straddle-mounted spool in accordance with an embodiment of the disclosure;

FIG. 10 is a schematic illustration of a high spool gas path with an overhung spool in accordance with an embodiment of the disclosure;

FIG. 11 is a graph illustrating commanded starter valve opening with respect to time in accordance with an embodiment of the disclosure; and

FIG. 12 is a graph illustrating a target rotor speed profile of a dry motoring profile and an actual rotor speed versus time in accordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are related to a bowed rotor start mitigation system in a gas turbine engine. Embodiments can include using a measured temperature value of the gas turbine engine to estimate heat stored in the engine core when a start sequence is initiated and identify a risk of a bowed rotor. The measured temperature value alone or in combination with other values can be used to calculate a bowed rotor risk parameter. For example, the measured temperature can be adjusted relative to an ambient temperature when calculating the bowed rotor risk parameter. The bowed rotor risk parameter may be used to take a control action to mitigate the risk of starting the gas turbine engine with a bowed rotor. The control action can include dry motoring, which may be performed by running an engine starting system at a lower speed with a longer duration than typically used for engine starting. Specifically, the dry motoring speed referred to herein is a speed below a major resonance speed where, if the temperatures are unhomogenized, the combination of the bowed rotor and similarly bowed casing and the resonance speed would lead to high amplitude oscillation in the rotor and high rubbing of blade tips on one side of the rotor, especially in the high pressure compressor if the rotor is straddle-mounted. The engine speed during dry motoring may oscillate within a motoring band of speed by modulating a starter valve, for example. Some embodiments increase the rotor speed of the starting spool to approach a critical rotor speed gradually according to a dry motoring profile and as thermal distortion is decreased they then accelerate beyond the critical rotor speed to complete the engine starting process.

A full authority digital engine control (FADEC) system or other system may send a message to the cockpit to inform the crew of an extended time start time due to bowed rotor mitigation actions prior to completing an engine start sequence. If the engine is in a ground test or in a test stand, a message can be sent to the test stand or cockpit based on the control-calculated risk of a bowed rotor. A test stand crew can be alerted regarding a requirement to keep the starting spool of the engine to a speed below the known resonance speed of the rotor in order to homogenize the temperature of the rotor and the casings about the rotor which also are distorted by temperature non-uniformity.

Monitoring of vibration signatures during the engine starting sequence can also or separately be used to assess the risk that a bowed rotor start has occurred due to some system malfunction and then direct maintenance, for instance, in the case of suspected outer air seal rub especially in the high compressor. Vibration data for the engine can also be monitored after bowed rotor mitigation is performed during an engine start sequence to confirm success of bowed rotor mitigation. If bowed rotor mitigation is unsuccessful or determined to be incomplete by the FADEC, resulting metrics (e.g., time, date, global positioning satellite (GPS) coordinates, vibration level vs. time, etc.) of the attempted bowed rotor mitigation can be recorded and/or transmitted to direct maintenance

During a dry motoring sequence, a starter air pressure valve can be modulated or otherwise dynamically adjusted to limit high rotor speed below high spool resonance speed and prevent rub during dry motoring operation. The bowed rotor risk parameter can also be used to limit dry motoring duration to reduce the impact on air starter turbine life. The monitoring of vibration signatures during the entire engine starting sequence can also or separately be used to assess the risk of a bowed rotor start and direct maintenance, for instance, in the case of suspected outer air seal rub especially in the high compressor.

Referring now to FIG. 1, a schematic illustration of a gas turbine engine 10 is provided. The gas turbine engine 10 has among other components a fan through which ambient air is propelled into the engine housing, a compressor for pressurizing the air received from the fan and a combustor wherein the compressed air is mixed with fuel and ignited for generating combustion gases. The gas turbine engine 10 further comprises a turbine section for extracting energy from the combustion gases. Fuel is injected into the combustor of the gas turbine engine 10 for mixing with the compressed air from the compressor and ignition of the resultant mixture. The fan, compressor, combustor, and turbine are typically all concentric about a central longitudinal axis of the gas turbine engine 10. Thus, thermal deflection of the components of the gas turbine engine 10 may create the aforementioned bowing or “bowed rotor” condition along the common central longitudinal axis of the gas turbine engine 10 and thus it is desirable to clear or remove the bowed condition prior to the starting or restarting of the gas turbine engine 10.

FIG. 1 schematically illustrates a gas turbine engine 10 that can be used to power an aircraft, for example. The gas turbine engine 10 is disclosed herein as a multi-spool turbofan that generally incorporates a fan section 22, a compressor section 24, a combustor section 26 and a turbine section 28. The fan section 22 drives air along a bypass flowpath while the compressor section 24 drives air along a core flowpath for compression and communication into the combustor section 26 then expansion through the turbine section 28. Although depicted as a turbofan gas turbine engine in the disclosed non-limiting embodiment with two turbines and is sometimes referred to as a two spool engine, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. In both of these architectures the starting spool is that spool that is located around the combustor, meaning the compressor part of the starting spool is flowing directly into the combustor and the combustor flows directly into the turbine section.

The engine 10 generally includes a low speed spool 30 and a high speed spool 32 mounted for rotation about an engine central longitudinal axis A relative to an engine static structure 36 via several bearing systems 38. It should be understood that various bearing systems 38 at various locations may alternatively or additionally be provided.

The low speed spool 30 generally includes an inner shaft 40 that interconnects a fan 42, a low pressure compressor 44 and a low pressure turbine 46. The inner shaft 40 is connected to the fan 42 through a geared architecture 48 to drive the fan 42 at a lower speed than the low speed spool 30 in the example of FIG. 1. The high speed spool 32 includes an outer shaft 50 that interconnects a high pressure compressor 52 and high pressure turbine 54. A combustor 56 is arranged between the high pressure compressor 52 and the high pressure turbine 54. A mid-turbine frame 57 of the engine static structure 36 is arranged generally between the high pressure turbine 54 and the low pressure turbine 46. The mid-turbine frame 57 further supports bearing systems 38 in the turbine section 28. The inner shaft 40 and the outer shaft 50 are concentric and rotate via bearing systems 38 about the engine central longitudinal axis A which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 then the high pressure compressor 52, mixed and burned with fuel in the combustor 56, then expanded over the high pressure turbine 54 and low pressure turbine 46. The mid-turbine frame 57 includes airfoils 59 which are in the core airflow path. The turbines 46, 54 rotationally drive the respective low speed spool 30 and high speed spool 32 in response to the expansion.

A number of stations for temperature and pressure measurement/computation are defined with respect to the gas turbine engine 10 according to conventional nomenclature. Station 2 is at an inlet of low pressure compressor 44 having a temperature T2 and a pressure P2. Station 2.5 is at an exit of the low pressure compressor 44 having a temperature T2.5 and a pressure P2.5. Station 3 is at an inlet of the combustor 56 having a temperature T3 and a pressure P3. Station 4 is at an exit of the combustor 56 having a temperature T4 and a pressure P4. Station 4.5 is at an exit of the high pressure turbine 54 having a temperature T4.5 and a pressure P4.5. Station 5 is at an exit of the low pressure turbine 46 having a temperature T5 and a pressure P5. Measured temperatures in embodiments may be acquired at one or more stations 2-5. For example, temperature T3 at station 3 can be used to as an engine rotor temperature measurement when there is no engine rotation. Alternatively, if available, temperature values at stations 4 (T4), 4.5 (T4.5), and/or 5 (T5) can be used as an engine rotor temperature. Temperature measurements can be normalized to account for hot day/cold day differences. For instance, temperature T2 can be used as an ambient temperature and a measured temperature (e.g., T3) can be normalized by subtracting temperature T2.

Although FIG. 1 depicts one example configuration, it will be understood that embodiments as described herein can cover a wide range of configurations. For example, embodiments may be implemented in a configuration that is described as a “straddle-mounted” spool 32A of FIG. 9. This configuration places two bearing compartments 37A and 39A (which may include a ball bearing and a roller bearing respectively), outside of the plane of most of the compressor disks of high pressure compressor 52A and at outside at least one of the turbine disks of high pressure turbine 54A. In contrast with a straddle-mounted spool arrangement, other embodiments may be implemented using an over-hung mounted spool 32B as depicted in FIG. 10. In over-hung mounted spool 32B, a bearing compartment 37B is located forward of the first turbine disk of high pressure turbine 54B such that the high pressure turbine 54B is overhung, and it is physically located aft of its main supporting structure. The use of straddle-mounted spools has advantages and disadvantages in the design of a gas turbine, but one characteristic of the straddle-mounted design is that the span between the bearing compartments 37A and 39A is long, making the amplitude of the high spot of a bowed rotor greater and the resonance speed that cannot be transited prior to temperature homogenization is lower. For any thrust rating, the straddle mounted arrangement, such as straddle-mounted spool 32A, gives Lsupport/Dhpt values that are higher, and the overhung mounted arrangement, such as overhung spool 32B, can be as much as 60% of the straddle-mounted Lsupport/Dhpt. Lsupport is the distance between bearings (e.g., between bearing compartments 37A and 39A or between bearing compartments 37B and 39B), and Dhpt is the diameter of the last blade of the high pressure turbine (e.g., high pressure turbine 54A or high pressure turbine 54B). As one example, a straddle-mounted engine starting spool, such as straddle-mounted spool 32A, with a roller bearing at bearing compartment 39A located aft of the high pressure turbine 54A may be more vulnerable to bowed rotor problems since the Lsupport/Dhpt ranges from 1.9 to 5.6. FIGS. 9 and 10 also illustrate a starter 120 interfacing via a tower shaft 55 with the straddle-mounted spool 32A proximate high compressor 52A and interfacing via tower shaft 55 with the overhung mounted spool 32B proximate high compressor 52B as part of a starting system.

Turning now to FIG. 2, a schematic of a starting system 100 for the gas turbine engine 10 of FIG. 1 is depicted according to an embodiment. The starting system 100 is also referred to generally as a gas turbine engine system. In the example of FIG. 2, the starting system 100 includes a controller 102 which may be an electronic engine control, such as a dual-channel FADEC, and/or engine health monitoring unit. In an embodiment, the controller 102 may include memory to store instructions that are executed by one or more processors. The executable instructions may be stored or organized in any manner and at any level of abstraction, such as in connection with a controlling and/or monitoring operation of the engine 10 of FIG. 1. The one or more processors can be any type of central processing unit (CPU), including a general purpose processor, a digital signal processor (DSP), a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. Also, in embodiments, the memory may include random access memory (RAM), read only memory (ROM), or other electronic, optical, magnetic, or any other computer readable medium onto which is stored data and control algorithms in a non-transitory form.

The starting system 100 can also include a data storage unit (DSU) 104 that retains data between shutdowns of the gas turbine engine 10 of FIG. 1. The DSU 104 includes non-volatile memory and retains data between cycling of power to the controller 102 and DSU 104. A communication link 106 can include an aircraft and/or test stand communication bus to interface with aircraft controls, e.g., a cockpit, various onboard computer systems, and/or a test stand.

A motoring system 108 is operable to drive rotation of a starting spool (e.g., high speed spool 32) of the gas turbine engine 10 of FIG. 1. Either or both channels of controller 102 can alternate on and off commands to an electromechanical device 110 coupled to a discrete starter valve 116A to achieve a partially open position of the discrete starter valve 116A to control a flow from a starter air supply 114 (also referred to as air supply 114) through a transfer duct 118 to an air turbine starter 120 (also referred to as starter 120 or pneumatic starter motor 120) to drive rotation of a starting spool of the gas turbine engine 10 below an engine idle speed. The air supply 114 (also referred to as starter air supply 114) can be provided by any known source of compressed air, such as an auxiliary power unit or ground cart.

The controller 102 can monitor a speed sensor, such as speed pickup 122 that may sense the speed of the engine rotor through its connection to a gearbox 124 which is in turn connected to the high speed spool 32 via tower shaft 55 (e.g., rotational speed of high speed spool 32) or any other such sensor for detecting or determining the speed of the gas turbine engine 10 of FIG. 1. The starter 120 may be coupled to the gearbox 124 of the gas turbine engine 10 of FIG. 1 directly or through a transmission such as a clutch system (not depicted). The controller 102 can establish a control loop with respect to rotor speed to adjust positioning of the discrete starter valve 116A.

The discrete starter valve 116A is an embodiment of a starter valve that is designed as an on/off valve which is typically commanded to either fully opened or fully closed. However, there is a time lag to achieve the fully open position and the fully closed position. By selectively alternating an on-command time with an off-command time through the electromechanical device 110, intermediate positioning states (i.e., partially opened/closed) can be achieved. The controller 102 can modulate the on and off commands (e.g., as a duty cycle using pulse width modulation) to the electromechanical device 110 to further open the discrete starter valve 116A and increase a rotational speed of the starting spool of the gas turbine engine 10 of FIG. 1. In an embodiment, the electromechanical device 110 has a cycle time defined between an off-command to an on-command to the off-command that is at most half of a movement time for the discrete starter valve 116A to transition from fully closed to fully open. Pneumatic lines 112A and 112B or a mechanical linkage (not depicted) can be used to drive the discrete starter valve 116A between the open position and the closed position. The electromechanical device 110 can be a solenoid that positions the discrete starter valve 116A based on intermittently supplied electric power as commanded by the controller 102. In an alternate embodiment, the electromechanical device 110 is an electric valve controlling muscle air to adjust the position of the discrete starter valve 116A as commanded by the controller 102.

In the example of FIG. 2, the starting system 100 also includes a vibration monitoring system 126. The vibration monitoring system 126 includes at least one vibration pickup 128, e.g., an accelerometer, operable to monitor vibration of the gas turbine engine 10 of FIG. 1. Vibration signal processing 130 can be performed locally with respect to the vibration pickup 128, within the controller 102, or through a separate vibration processing system, which may be part of an engine health monitoring system to acquire vibration data 132. Alternatively, the vibration monitoring system 126 can be omitted in some embodiments.

One or more temperature sensors 134, such as thermocouples, can provide measured temperatures at associated locations of the gas turbine engine 10 to the controller 102. For example, the temperature sensors 134 can be located at station 2 (T2), station 2.5 (T2.5), station 3 (T3), station 4 (T4), station 4.5 (T4.5), and/or station 5 (T5) as previously described with respect to FIG. 1.

Similar to FIG. 2, FIG. 3 is a schematic illustration of a starting system 100A for the gas turbine engine 10 of FIG. 1 in accordance with another embodiment. The starting system 100A includes controller 102 that controls motoring system 108A, as an alternate embodiment of the motoring system 108 of FIG. 2. Rather than using an electromechanical device 110 coupled to a discrete starter valve 116A to achieve a partially open position of the discrete starter valve 116A of FIG. 2, the motoring system 108A of FIG. 3 uses a variable position starter valve 116B. In FIG. 3, either or both channels of controller 102 can output a valve control signal 150 operable to dynamically adjust a valve angle of the variable position starter valve 116A that selectively allows a portion of the air supply 114 to pass through the variable position starter valve 116B and transfer duct 118 to air turbine starter 120. The variable position starter valve 116B is a continuous/infinitely adjustable valve that can hold a commanded valve angle, which may be expressed in terms of a percentage open/closed and/or an angular value (e.g., degrees or radians). Performance parameters of the variable position starter valve 116B can be selected to meet dynamic response requirements of the starting system 100A. For example, in some embodiments, the variable position starter valve 116B has a response rate of 0% to 100% open in less than 40 seconds. In other embodiments, the variable position starter valve 116B has a response rate of 0% to 100% open in less than 30 seconds. In further embodiments, the variable position starter valve 116B has a response rate of 0% to 100% open in less than 20 seconds.

The controller 102 can monitor a valve angle of the variable position starter valve 116B using valve angle feedback signals 152 provided to both channels of controller 102. As one example, in an active/standby configuration, both channels of the controller 102 can use the valve angle feedback signals 152 to track a current valve angle, while only one channel designated as an active channel outputs valve control signal 150. Upon a failure of the active channel, the standby channel of controller 102 can take over as the active channel to output valve control signal 150. In an alternate embodiment, both channels of controller 102 output all or a portion of a valve angle command simultaneously on the valve control signals 150. The controller 102 can establish an outer control loop with respect to rotor speed and an inner control loop with respect to the valve angle of the variable position starter valve 116B. One or more temperature sensors 134, such as thermocouples, can provide measured temperatures at associated locations of the gas turbine engine 10 to the controller 102.

As in the example of FIG. 2, the starting system 100A of FIG. 3 also includes vibration monitoring system 126. The vibration monitoring system 126 includes at least one vibration pickup 128, e.g., an accelerometer, operable to monitor vibration of the gas turbine engine 10 of FIG. 1. Vibration signal processing 130 can be performed locally with respect to the vibration pickup 128, within the controller 102, or through a separate vibration processing system, which may be part of an engine health monitoring system to acquire vibration data 132. Alternatively, the vibration monitoring system 126 can be omitted in some embodiments.

FIG. 4 is a block diagram of a system 200 for bowed rotor start mitigation using direct temperature measurement that may control the discrete starter valve 116A of FIG. 2 or the variable position starter valve 116B of FIG. 3 via control signals 210 in accordance with an embodiment. The system 200 may also be referred to as a bowed rotor start mitigation system. In the example of FIG. 4, the system 200 includes sensor processing 202 operable to acquire and condition data from a variety of sensors such as the speed pickup 122, vibration pickup 128, and temperature sensors 134 of FIG. 2. Where the variable position starter valve 116B of FIG. 3 is used, the sensor processing 202 may also provide valve angle 207 to motoring controller 208 based on the valve angle feedback 152 of FIG. 3. In the example of FIG. 4, sensor processing 202 provides compressor exit temperature T3 and ambient temperature T2 to core temperature processing 204. Alternatively or additionally, one or more temperature values from stations 4 (T4), 4.5 (T4.5), and/or 5 (T5) can be provided to core temperature processing 204. Sensor processing 202 can provide rotor speed N2 (i.e., speed of high speed spool 32) to a motoring controller 208 and a mitigation monitor 214. Sensor processing 202 may also provide vibration data 132 to mitigation monitor 214. The sensor processing 202, core temperature processing 204, motoring controller 208, and/or mitigation monitor 214 may be part of controller 102.

The heat state of the engine 10 or T_(core) is determined by the core temperature processing 204. When the gas turbine engine 10 has stopped rotating (e.g., rotor speed N2 is zero), the compressor exit temperature T3 may be substantially equal to T_(core). In some embodiments, T_(core) is set equal to T3-T2 to adjust the temperature with respect to the measured ambient temperature of the gas turbine engine 10. Further, temperature readings from other stations of the gas turbine engine 10 can be used to determine T_(core). Communication link 106 can provide the core temperature processing 204 with an indication 205 that a start sequence of the gas turbine engine 10 has been initiated. Once rotation of the gas turbine engine 10 begins, temperature readings can be collected for a predetermined period of time, such as ten seconds. The temperature readings, e.g., T3 or T3-T2, can be averaged as core temperature T_(core) before the temperature starts to change due to air flow from engine rotation. The core temperature processing 204 can determine a bowed rotor risk parameter that is based on the measured temperature using a mapping function or lookup table. The bowed rotor risk parameter can have an associated motoring time defining an anticipated amount of time for the motoring controller 208 to send control signals 210 to electromechanical device 110 for controlling discrete starter valve 116A of FIG. 2 or a dry motoring profile to control valve control signals 150 for controlling variable position starter valve 116B of FIG. 3. For example, a higher risk of a bowed rotor may result in a longer duration of dry motoring or an extended dry motoring profile to reduce a temperature gradient prior to starting the gas turbine engine 10 of FIG. 1.

The bowed rotor risk parameter may be quantified according to a profile curve 402 selected from a family of curves 404 that align with observed aircraft/engine conditions that impact turbine bore temperature (e.g., starting spool temperature) and the resulting bowed rotor risk as depicted in the example graph 400 of FIG. 6.

As used herein, motoring of the engine 10 in a modified start sequence refers to the turning of a starting spool by the starter 120 at a reduced speed without introduction of fuel and an ignition source in order to cool the engine 10 to a point wherein a normal start sequence can be implemented without starting the engine 10 in a bowed rotor state. In other words, cool or ambient air is drawn into the engine 10 while motoring the engine 10 at a reduced speed in order to clear the “bowed rotor” condition, which is referred to as a dry motoring mode.

The motoring controller 208 uses a dynamic control calculation in order to determine a required valve position of the starter valve 116A, 116B used to supply an air supply or starter air supply 114 to the engine 10 in order to limit the motoring speed of the engine 10 to a target speed N_(target) within a motoring band or following a dry motoring profile due to the position of the starter valve 116A, 116B. The required valve position of the starter valve 116A, 116B can be determined based upon an air supply pressure as well as other factors including but not limited to ambient air temperature, parasitic drag on the engine 10 from a variety of engine driven components such as electric generators and hydraulic pumps, and other variables such that the motoring controller 208 closes the loop for an engine motoring speed target N_(target) for the required amount of time based on the output of the bowed rotor start risk model 206. In one embodiment, the dynamic control of the valve position (e.g., open state of the valve (e.g., fully open, ½ open, ¼ open, etc.) in order to limit the motoring speed of the engine 10) is controlled via duty cycle control (on/off timing using pulse width modulation) of electromechanical device 110 for discrete starter valve 116A.

Referring now to FIG. 7, a graph 450 illustrating starting profiles of a normal start 452 and a bowed rotor mitigation sequence start 454 is depicted according to an embodiment. During the normal start 452, upon receiving the indication 205 of FIG. 4 that a start sequence of the gas turbine engine 10 has been initiated, the controller 102 of FIG. 2 opens discrete starter valve 116A of FIG. 2 (or variable position starter valve 116B of FIG. 3) to enable the air supply 114 to drive rotation of starter 120 to get rotor speed N2 of the gas turbine engine 10 up to idle speed 456. In an embodiment, upon receiving the indication 205 of FIG. 4, controller 102 monitors one or more temperature sensors 134 for a predetermined period of time 458 while rotor speed N2 is at or about zero RPM. If core temperature processing 204 of FIG. 4 determines that bowed rotor mitigation is needed based on measured temperature, then bowed rotor mitigation sequence start 454 is performed instead of normal start 452.

In the example of FIG. 7, bowed rotor mitigation sequence start 454 oscillates the rotor speed N2 within a motoring band 460 for a motoring time. The motoring band 460 can be defined between a starter valve close limit 462 (i.e., an upper N2 speed limit) and a starter valve re-open limit 464 (i.e., a lower N2 limit). In the example of FIG. 7, the starter valve close limit 462 is about 3000 RPM and the starter valve re-open limit 464 is about 2000 RPM. The actual values used for the starter valve close limit 462 and the starter valve re-open limit 464 can be tuned to the specific performance characteristics of the gas turbine engine 10 as further described with respect to FIG. 8.

In some embodiments, an anticipated amount of dry motoring time can be used to determine a target rotor speed profile in a dry motoring profile for the currently observed conditions. As one example, one or more baseline characteristic curves for the target rotor speed profile can be defined in tables or according to functions that may be rescaled to align with the observed conditions. An example of a target rotor speed profile 1002 is depicted in graph 1000 of FIG. 12 that includes a steep initial transition portion 1004, followed by a gradually increasing portion 1006, and a late acceleration portion 1008 that increases rotor speed above a critical rotor speed, through a fuel-on speed and an engine idle speed. The target rotor speed profile 1002 can be rescaled with respect to time and/or select portions (e.g., portions 1004, 1006, 1008) of the target rotor speed profile 1002 can be individually or collectively rescaled (e.g., slope changes) with respect to time to extend or reduce the total motoring time. The target rotor speed profile 1002 may include all positive slope values such that the actual rotor speed 1010 is driven to essentially increase continuously while bowed rotor start mitigation is active. While the example of FIG. 12 depicts one example of the target rotor speed profile 1002 that can be defined in a dry motoring profile, it will be understood that many variations are possible in embodiments.

The example of FIG. 11 illustrates how a valve angle command 902 can be adjusted between 0 to 100% of a commanded starter valve opening to generate the actual rotor speed 1010 of FIG. 12. As the actual rotor speed 1010 tracks to the steep initial transition portion 1004 of the target rotor speed profile 1002, the valve angle command 902 transitions through points “a” and “b” to fully open the variable position starter valve 116B. As the slope of the target rotor speed profile 1002 is reduced in the gradually increasing portion 1006, the valve angle command 902 is reduced between points “b” and “c” to prevent the actual rotor speed 1010 from overshooting the target rotor speed profile 1002. In some embodiments, decisions to increase or decrease the commanded starter valve opening is based on monitoring a rate of change of the actual rotor speed 1010 and projecting whether the actual rotor speed 1010 will align with the target rotor speed profile 1002 at a future time. If it is determined that the actual rotor speed 1010 will not align with the target rotor speed profile 1002 at a future time, then the valve angle of the variable position starter valve 116B is adjusted (e.g., increase or decrease the valve angle command 902) at a corresponding time. In the example of FIGS. 11 and 12, the valve angle command 902 oscillates with a gradually reduced amplitude between points “c”, “d”, and “e” as the actual rotor speed 1010 tracks to the target rotor speed profile 1002 through the gradually increasing portion 1006. As dry motoring continues, the overall homogenization of the engine 10 increases, which allows the actual rotor speed 1010 to safely approach the critical rotor speed without risking damage. The valve angle command transitions from point “e” to point “f” and beyond to further increase the actual rotor speed 1010 in the late acceleration portion 1008 above the critical rotor speed, through a fuel-on speed and an engine idle speed. By continuously increasing the actual rotor speed 1010 during dry motoring, the bowed rotor condition can be reduced faster than holding a constant slower speed.

In reference to FIGS. 4 and 12, the mitigation monitor 214 of FIG. 4 can operate in response to receiving a complete indicator 212 to run a verification of the bowed rotor mitigation. The mitigation monitor 214 can provide mitigation results 216 to the motoring controller 208 and may provide result metrics 218 to other systems, such a maintenance request or indicator. Peak vibrations can be checked by the mitigation monitor 214 during the start processes to confirm that bowed rotor mitigation successfully removed the bowed rotor condition. The mitigation monitor 214 may also run while dry motoring is active to determine whether adjustments to the dry motoring profile are needed. For example, if a greater amount of vibration is detected than was expected, the mitigation monitor 214 can request that the motoring controller 208 reduce a slope of the target rotor speed profile 1002 of FIG. 12 to extend the dry motoring time before driving the actual rotor speed 1010 of FIG. 12 up to the critical rotor speed. Similarly, if the magnitude of vibration observed by the mitigation monitor 214 is less than expected, the mitigation monitor 214 can request that the motoring controller 208 increase a slope of the target rotor speed profile 1002 of FIG. 12 to reduce the dry motoring time before driving the actual rotor speed 1010 of FIG. 12 up to the critical rotor speed.

FIG. 5 is a flow chart illustrating a method 300 of bowed rotor start mitigation using direct temperature measurement of a gas turbine engine in accordance with an embodiment. The method 300 of FIG. 5 is described in reference to FIGS. 1-12 and may be performed with an alternate order and include additional steps. At block 302, the controller 102 receives receive a speed input indicative of a rotor speed (e.g., N2) of the gas turbine engine 10. The speed input may be directly or indirectly indicative of the rotational speed of high speed spool 32, for instance, derived from a rotational speed of gearbox 124, from speed pickup 122, or another source (not depicted).

At block 304, the controller 102 receives a measured temperature from a temperature sensor 134. The measured temperature can be determined based on reading one or more temperature sensors 134 of the gas turbine engine 10 for a predetermined period of time when a start sequence of the gas turbine engine is initiated (e.g., based on indication 205). The measured temperature may be adjusted with respect to a measured ambient temperature of the gas turbine engine 10 (e.g., T3-T2, T4-T2, etc.).

At block 306, bowed rotor start mitigation can be performed using motoring system 108, 108A to rotate a starting spool of the gas turbine engine 10. The controller 102 can drive motoring of the gas turbine engine 10 by controlling the motoring system 108, 108A to oscillate the rotor speed (e.g., N2) within a motoring band for a motoring time based on the measured temperature when a start sequence of the gas turbine engine 10 is initiated. The motoring band can include a range of speeds below a resonance speed of the gas turbine engine 10. The motoring time can be associated with a bowed rotor risk parameter that is determined based on the measured temperature. Alternatively, other motoring profiles can be used, such as the target rotor speed profile 1002 for controlling starting spool speed during dry motoring.

At block 308, based on determining that bowed rotor mitigation is complete, the controller 102 can monitor the vibration level of the gas turbine engine 10 while sweeping through a range of rotor speeds including a critical rotor speed and determine whether bowed rotor mitigation was successful prior to starting the gas turbine engine 10. The mitigation monitor 214 may receive a complete indicator 212 from the motoring controller 208 when the motoring controller 208 has completed dry motoring, for instance, if the motoring time has elapsed. If the mitigation monitor 214 determines that the bowed rotor condition still exists based on vibration data 132 collected, the motoring controller 208 may restart dry motoring, or a maintenance request or indicator can be triggered along with providing result metrics 218 for further analysis. Metrics of attempted bowed rotor mitigation can be recorded in the DSU 104 based on determining that the attempted bowed rotor mitigation was unsuccessful or incomplete.

FIG. 8 is a graph illustrating examples of various vibration level profiles 502 of an engine, such as gas turbine engine 10 of FIG. 1. The vibration level profiles 502 represent a variety of possible vibration levels observed before and/or after performing bowed rotor mitigation. A critical speed 510 is the speed at which a vibration peak is expected due to amplification effects of a bowed rotor condition along with other contributions to vibration level generally. A peak vibration 504 at critical rotor speed 510 may be used to trigger different events. For example, if the peak vibration 504 at critical rotor speed 510 is below a maintenance action threshold 506, then no further actions may be needed. If the peak vibration 504 at critical rotor speed 510 is above a damage risk threshold 508, then an urgent maintenance action may be requested such as an engine check. If the peak vibration 504 at critical rotor speed 510 is between the maintenance action threshold 506 and the damage risk threshold 508, then further bowed rotor mitigation actions may be requested, such as extending/restarting dry motoring. In one embodiment, a maintenance request is triggered based on the actual vibration level exceeding maintenance action threshold 506 after completing an attempt of bowed rotor mitigation.

The lowest rotor vibration vs. speed in FIG. 8 is for a fully homogenized rotor, where mitigation is not necessary (engine parked all night long, for example). The next higher curve shows a mildly bowed rotor and so on. The maintenance action threshold 506 is a threshold for setting a maintenance flag such as requiring a troubleshooting routine of one or more system elements. The damage risk threshold 508 may be a threshold to trigger a more urgent maintenance requirement up to and including an engine check.

Further, as can be seen in the example of FIG. 8, the motoring band 460 is defined for a range of N2 rotor speeds below the critical speed 510 and such that the worst case expected vibration 512 is below the maintenance action threshold 506 within the motoring band 460. Performing motoring at a higher speed (e.g., 2000-3000 RPM) but less than the critical speed 510 can reduce the time needed for the temperature homogenization process while also avoiding vibrations reaching potentially harmful levels.

Accordingly and as mentioned above, it is desirable to detect, prevent and/or clear a “bowed rotor” condition in a gas turbine engine that may occur after the engine has been shut down. As described herein and in one non-limiting embodiment, the controller 102 may be programmed to automatically take the necessary measures in order to provide for a modified start sequence without pilot intervention other than the initial start request. In an exemplary embodiment, the controller 102 and/or DSU 104 comprises a microprocessor, microcontroller or other equivalent processing device capable of executing commands of computer readable data or program for executing a control algorithm and/or algorithms that control the start sequence of the gas turbine engine. In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the execution of Fourier analysis algorithm(s), the control processes prescribed herein, and the like), the controller 102 and/or DSU 104 may include, but not be limited to, a processor(s), computer(s), memory, storage, register(s), timing, interrupt(s), communication interfaces, and input/output signal interfaces, as well as combinations comprising at least one of the foregoing. For example, the controller 102 and/or DSU 104 may include input signal filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. As described above, exemplary embodiments of the disclosure can be implemented through computer-implemented processes and apparatuses for practicing those processes.

While the present disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the present disclosure is not limited to such disclosed embodiments. Rather, the present disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the present disclosure. Additionally, while various embodiments of the present disclosure have been described, it is to be understood that aspects of the present disclosure may include only some of the described embodiments. Accordingly, the present disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

The invention claimed is:
 1. A bowed rotor start mitigation system for a gas turbine engine, the bowed rotor start mitigation system comprising: a controller operable to receive a speed input indicative of a rotor speed of the gas turbine engine and a measured temperature of the gas turbine engine, the controller further operable to drive motoring of the gas turbine engine by oscillating the rotor speed within a motoring band for a motoring time based on the measured temperature when a start sequence of the gas turbine engine is initiated, wherein the motoring time is associated with a bowed rotor risk parameter that is determined based on the measured temperature using a mapping function or lookup table with a higher risk resulting in a longer duration to reduce a temperature gradient within the gas turbine engine.
 2. The bowed rotor start mitigation system as in claim 1, wherein the motoring band comprises a range of speeds below a resonance speed of the gas turbine engine.
 3. The bowed rotor start mitigation system as in claim 1, wherein the measured temperature is adjusted with respect to a measured ambient temperature of the gas turbine engine by subtracting the measured ambient temperature.
 4. The bowed rotor start mitigation system as in claim 1, wherein the measured temperature is determined based on averaging readings from one or more temperature sensors of the gas turbine engine for a predetermined period of time when the start sequence of the gas turbine engine is initiated.
 5. The bowed rotor start mitigation system as in claim 1, wherein based on determining that bowed rotor mitigation is complete, the controller is operable to monitor a vibration level while sweeping through a range of rotor speeds including a critical rotor speed and determine whether bowed rotor mitigation was successful prior to starting the gas turbine engine.
 6. The bowed rotor start mitigation system as in claim 1, wherein the measured temperature is determined based on averaging data from one or more temperature sensors at station 3 of the gas turbine engine.
 7. The bowed rotor start mitigation system as in claim 1, wherein the measured temperature is determined based on averaging data from one or more temperature sensors at station 4 of the gas turbine engine.
 8. The bowed rotor start mitigation system as in claim 1, wherein the measured temperature is determined based on averaging data from one or more temperature sensors at station 4.5 of the gas turbine engine.
 9. The bowed rotor start mitigation system as in claim 1, wherein the controller is operable to drive motoring of the gas turbine engine by oscillating the rotor speed to continuously rotate within the motoring band for the motoring time based on the measured temperature when the start sequence of the gas turbine engine is initiated and prior to determining that bowed rotor mitigation is complete.
 10. A gas turbine engine system comprising: a motoring system operable to drive rotation of the gas turbine engine; a speed pickup; a temperature sensor; and an electronic engine control operable to receive a speed input from the speed pickup indicative of a rotor speed of the gas turbine engine and a measured temperature from the temperature sensor, the electronic engine control further operable to drive motoring of the gas turbine engine by controlling the motoring system to oscillate the rotor speed within a motoring band for a motoring time based on the measured temperature when a start sequence of the gas turbine engine is initiated, wherein the motoring time is associated with a bowed rotor risk parameter that is determined based on the measured temperature using a mapping function or lookup table with a higher risk resulting in a longer duration to reduce a temperature gradient within the gas turbine engine.
 11. The gas turbine engine as in claim 10, wherein the motoring band comprises a range of speeds below a resonance speed of the gas turbine engine and the measured temperature is adjusted with respect to a measured ambient temperature of the gas turbine engine by subtracting the measured ambient temperature.
 12. A method of bowed rotor start mitigation for a gas turbine engine, the method comprising: receiving, by a controller, a speed input indicative of a rotor speed of the gas turbine engine; receiving, by the controller, a measured temperature of the gas turbine engine; and driving, by the controller, motoring of the gas turbine engine by oscillating the rotor speed within a motoring band for a motoring time based on the measured temperature when a start sequence of the gas turbine engine is initiated, wherein the motoring time is associated with a bowed rotor risk parameter that is determined based on the measured temperature using a mapping function or lookup table with a higher risk resulting in a longer duration to reduce a temperature gradient within the gas turbine engine.
 13. The method as in claim 12, wherein the motoring band comprises a range of speeds below a resonance speed of the gas turbine engine.
 14. The method as in claim 12, wherein the measured temperature is adjusted with respect to a measured ambient temperature of the gas turbine engine by subtracting the measured ambient temperature.
 15. The method as in claim 12, wherein the measured temperature is determined based on averaging readings from one or more temperature sensors of the gas turbine engine for a predetermined period of time when the start sequence of the gas turbine engine is initiated.
 16. The method as in claim 12, wherein based on determining that bowed rotor mitigation is complete, the controller is operable to monitor a vibration level while sweeping through a range of rotor speeds including a critical rotor speed and determine whether bowed rotor mitigation was successful prior to starting the gas turbine engine.
 17. The method as in claim 12, wherein the measured temperature is determined based on averaging data from one or more temperature sensors at station 3 of the gas turbine engine.
 18. The method as in claim 12, wherein the measured temperature is determined based on averaging data from one or more temperature sensors at station 4 of the gas turbine engine.
 19. The method as in claim 12, wherein the measured temperature is determined based on averaging data from one or more temperature sensors at station 4.5 of the gas turbine engine.
 20. The method of claim 12, wherein driving motoring of the gas turbine engine is performed by oscillating the rotor speed to continuously rotate within the motoring band for the motoring time based on the measured temperature when the start sequence of the gas turbine engine is initiated and prior to determining that bowed rotor mitigation is complete. 